1. Field of the Invention
The present invention relates to a substrate for use in mass spectrometry including a process of desorbing and ionizing an object substance to be measured by using a primary beam selected from ions, neutral particles, electrons, and a laser beam, and also to a mass spectrometry method.
Furthermore, the present invention also relates to imaging detection of constituents of each kind constituting a measurement object, in particular organic substances such as proteins, with a mass spectrometry device including a process of desorbing and ionizing an object substance to be measured by using a primary beam selected from ions, neutral particles, electrons, and a laser beam.
2. Description of the Related Art
In a mass spectrometry device, an object substance to be measured is ionized by some method, an electric field or a magnetic field is applied to the ionized substance, separation is performed according to a mass/charge ratio (m/z), and then the measurement object is qualitatively and quantitatively analyzed from an electrically detected mass spectrum. In this case, a variety of ionization methods are used, such as electron spray ionization (ESI), electron bombardment ionization (EI), chemical ionization (CI), fast atom bombardment (FAB), field desorption (FD), laser desorption ionization (LDI), matrix assisted laser desorption ionization (MALDI), and secondary ion mass spectrometry (SIMS) in which irradiation is performed with elemental ions, element cluster ions, and molecular ions. For example, in laser ionization mass spectrometer, a mass spectrum and the like can be measured by irradiating and ionizing a sample with a pulsed laser beam and introducing the ions into an analytical unit, for example, of a time of flight type.
To enable mass spectrometry of the object substance to be measured, a state has to be formed in which the substance contained in the analyte is an independent molecular unit, and this independent molecular unit has to have a positive or negative electric charge. Among the above-described mass spectrometry methods, the MALDI method has found especially broad application in a variety of fields in recent years because this method makes it possible to measure molecules with a high molecular weight such as polymer materials and proteins that have been heretofore difficult to measure. This is apparently because the MALDI method makes it possible to satisfy the two above-described conditions enabling mass spectrometry since the method uses a matrix that weakens interaction between the molecules to be measured and, therefore, increases the extraction efficiency of components to be measured as independent molecular units and also since the matrix itself can perform ionization of the molecules that are the measurement object by a reaction induced by laser irradiation.
Examples of the substance to be measured that is provided with an electric charge include radical cations obtained by pulling electrons off the substance to be measured, radical anions obtained by donating electrons to the substance to be measured, cations obtained by donating a proton or a cation of an alkali metal or silver to the substance to be measured, and anions obtained by donating an anion of a halogen or the like or by deprotonizing. In particular, in biomolecules such as proteins, a large number of polar groups are present and mass spectrometry can be conducted with a comparatively high sensitivity by cationization based on addition of protons.
In the field of mass spectrometry using laser irradiation, porous silicon has been used in recent years instead of a matrix, thereby making it possible to perform mass spectrometry with a comparatively good sensitivity and in a state in which peaks of impurities derived from the matrix are small, and this approach attracted much attention. Although the operation effect of mass spectrometry using a porous substrate is unclear, apparently because the specific surface area is larger than that of a flat substrate, the number of adsorption points of the analyte molecules is large and the degree of aggregation of these molecules on the substrate is decreased, thereby increasing the ratio of desorption in single molecular units by laser irradiation.
Furthermore, imaging technology using mass spectrometry has also attracted much attention in recent years. This is because a strong demand arose for specifying the location of developed proteins or impurities that adhered to the surface, for example, in biological tissues such as tumor cells and electronic materials such as semiconductor wafers.
In imaging technology based on mass spectrometry, a process of desorbing and ionizing the analyte molecules is carried out by a device using irradiation with a focused ion beam or laser beam. In particular, in SIMS, molecules that have adhered to the surface can be desorbed with a very high efficiency by irradiation with gallium ions or gold ions. As for the molecules with a comparatively high molecular weight that are difficult to desorb, because such molecules can be fragmented during irradiation with gallium ions or gold ions, it is still possible to obtain information, even though partial, that relates to the molecules that are the measurement object.
In mass spectrometry of such a type that uses irradiation with laser or with gallium ions or gold ions, the desorption of molecules can also proceed in a state of neutral molecules or neutral radicals, rather than only in a state of ions. Detection in mass spectrometers is performed on the basis of charge information of molecules that are desorbed in monomolecular units. The resultant problem is that neutral molecules or radicals cannot be detected even when the desorbed number thereof is large.
The problem associated with ionization efficiency of the molecules to be measured becomes particularly serious in mass spectrometry in which irradiation is performed with a laser or gadolinium ions, without using a matrix.
To resolve this problem, examples of Japanese Patent Laid-open No. 2006-201042 discloses a method for adding sodium iodide to the molecules to be measured and detecting the molecules as adducts of sodium ions. Furthermore, US Patent Application Publication No. 2006/0118711 (corresponding to Japanese Patent Laid-open No. 2006-153493) discloses a method for increasing ionization efficiency by adding an acid such as trifluoroacetic acid, hydrochloric acid, nitric acid, and hydrofluoric acid.
However, although the addition of a metal salt such as an alkali metal salt sometimes makes it possible to ionize the molecules that are the measurement object with good efficiency, such a salt is also known to inhibit ionization, as disclosed in Japanese Patent Laid-open No. 2006-170857, and is not necessarily effective in increasing the ionization efficiency. Furthermore, adding an acid such as trifluoroacetic acid or hydrochloric acid can be effective because the acid has a proton donating capacity and produces no ionization inhibiting effect like metal ions. However, these acids have high volatility. In particular, because of high-vacuum state inside a mass spectrometer, these volatile acids can be volatilized during measurement and the proton donating capacity thereof can change. In measurements performed in a plurality of locations for imaging, the concentration of acid differs depending on the measurement site or measurement order, and this difference can change the ionization efficiency. By contrast, sulfuric acid is known as a non-volatile acid, but where a solvent such as water contained in the measurement sample evaporates and the concentration of sulfuric acid increases, there is a risk of modifying the molecules that are the measurement object by a strong oxidizing or dehydrating reaction of sulfuric acid.